Apparatus and methods for symbol timing error detection, tracking and correction

ABSTRACT

Systems and methods for adjusting timing in a communication system, such as an OFDM system are described. In one implementation an error signal is generated to adjust the timing of a variable rate interpolator so as to adjust FFT timing. The error signal may be based on detection of significant peaks in an estimate of the impulse response of the channel, with the peak locations being tracked over subsequent symbols and the system timing adjusted in response to changes in the peaks.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/261,659 entitled APPARATUS &METHODS FOR SYMBOL TIMING ERROR DETECTION, TRACKING AND CORRECTION,filed on Nov. 16, 2009, the content of which is hereby incorporated byreference herein in its entirety for all purposes.

FIELD

This disclosure relates generally to communications systems, includingorthogonal frequency division multiplexing (OFDM) systems. Moreparticularly, but not exclusively, the disclosure relates to apparatusand methods for determining and tracking received symbol timing in acommunication receiver, as well as providing adjustments for controllingsymbol detection timing so as to maximize received symbol energy and/orminimize inter-symbol interference (ISI).

BACKGROUND

Orthogonal frequency division multiplexed (OFDM) communications systemshave been developed to address problems in high data rate communicationssystems, such as multipath interference. In an OFDM system, atransmitter module receives an incoming data stream and modulates thedata on orthogonal frequency domain sub-carriers. The modulatedsub-carriers are then sent as an OFDM symbol to a receiver. In many OFDMsystems a cyclic prefix (CP) is added to the OFDM symbol in thetransmitter, typically by inserting a repeat of the end of the symbol ina guard interval at the front of the symbol. By dividing the incomingdata stream among multiple sub-carriers, the data rate and thus thebandwidth of these individual sub-carriers is decreased relative to thebandwidth of the incoming data stream. The resulting increase in theduration of the data symbols associated with each sub-carrier candecrease the impact of multipath interference and associatedinter-symbol interference (ISI).

One implementation advantage of OFDM systems is that Fast FourierTransforms (FFTs) and Inverse Fast Fourier Transforms (IFFTs) can beused to simplify the demodulation and modulation processes,respectively. At an OFDM transmitter, incoming data signals are firstdemultiplexed into a plurality “N” of data sub-signals, each having alower data rate than the incoming data. Each sub-signal is thentranslated in parallel into corresponding frequency domain symbols in areal or complex signal constellation (with example constellations usingmodulation such as QPSK or QAM). An IFFT bank can then be used toconvert the frequency domain symbols into complex time-domain I and Qsignals at a appropriate baseband or IF frequency, which are thencombined to generate a transmit symbol. The transmit symbol can then beupconverted and transmitted at a desired transmit frequency.

At the receiver, an inverse process is applied to the incoming signal.In particular, the received time-domain signal may be quadrature mixedto generate I and Q signals, which are typically at baseband. Thebaseband signals are then sampled and digitized using analog-to-digitalconverters (“ADCs”). A forward FFT then operates upon a set of sampleswithin a “window” of the composite received signal in order to convertthe received signal back to a plurality of N parallel sub-carrier symbolstreams, each of which is then converted to a corresponding binary datastream. These streams are then remultiplexed into a serial stream, whichis an estimate of the incoming data stream provided to the transmitter.

However, the existence of Doppler and multipath conditions within thecommunication channel between the transmitter and receiver can impairthe integrity of the transmit signal. Although the use of a cyclicprefix can partially mitigate the adverse effects of such degradedchannel conditions, these conditions can cause shifting of the optimallocation of the window used for FFT sampling. Failure to appropriatelyposition this FFT window can reduce the quality of the signal producedby the receiver.

SUMMARY

The present invention relates generally to communications systems,including orthogonal frequency division multiplexing (OFDM) systems.More particularly, but not exclusively, the invention relates toapparatus and methods for determining and tracking received symboltiming in a communication receiver, as well as providing adjustments forcontrolling symbol detection timing so as to maximize received symbolenergy and/or minimize intersymbol interference. Although theembodiments described herein are illustrated in the context of an OFDMcommunications system and OFDM receiver, it is noted that embodiments ofthe invention may also be applied to other types of communicationssystems in addition to OFDM systems.

In one aspect, the disclosure relates to a method for controlling FFTmodule timing in a receiver. The method may include, for example, one ormore of the stages of setting an initial timing of the FFT module basedon an initial FFT timing parameter, determining, based at least in parton a channel impulse response estimate associated with a first receivedsymbol, an FFT timing adjustment parameter, and adjusting the initialFFT timing based at least in part on the FFT timing adjustment parameterso as to provide an adjusted FFT timing.

The initial FFT timing may be adjusted, for example, by providing aresampled FFT input signal based on the received signal. The sample rateof the resampled signal may be set based at least in part on the FFTtiming adjustment parameter. The FFT timing adjustment parameter may bean error signal. A variable rate interpolator may be configured togenerate the resampled FFT signal at a sample rate that may be based atleast in part on the FFT timing adjustment parameter. The FFT timingadjustment parameter may be an error signal.

The stage of determining an FFT timing adjustment parameter may includeone or more of the steps of receiving the channel impulse responseestimate, determining one or more peaks in the channel impulse responseestimate, comparing the one or more peaks to a set of one or morereference values, and generating the FFT timing adjustment parameterbased at least in part on said comparing.

The one or more reference values may be generated by a process that mayinclude one or more stages of setting a threshold value, determining oneor more pairs of threshold crossing points, generating the one or morepeaks based on the one or more pairs of threshold crossing points, andstoring the one or more peaks as the reference values.

The stage of determining one or more peaks may include one or more ofthe stages of receiving the channel impulse response estimate,determining one or more sets of threshold crossing points, andgenerating the one or more peaks based on the one or more sets ofthreshold crossing points. The ones of said one or more peaks may begenerated as the average of corresponding ones of the threshold crossingpoints of the one or more sets of threshold crossing points.

The stage of generating the FFT timing adjustment parameter based atleast in part on said comparing may include one or both of determining asubset of peaks within said one or more peaks, said subset consisting ofpeaks within a distance WIN of ones of the reference points, andgenerating the FFT timing adjustment parameter as a function of thedifferences between ones of the subset of peaks and corresponding onesof the reference points. The method may further include generating anupdated FFT timing adjustment responsive to receipt of a subsequentsymbol at the receiver

In another aspect, the disclosure relates to a communication apparatus.The communication apparatus may include a processor module configured toperform one or more of the stages of setting an initial timing of an FFTmodule based on an initial FFT timing parameter, determining, based atleast in part on a channel impulse response estimate associated with afirst received symbol, an FFT timing adjustment parameter, and adjustingthe initial FFT timing based at least in part on the FFT timingadjustment parameter so as to provide an adjusted FFT timing.

The communications apparatus may include one or more of a means to setan initial timing of an FFT module based on an initial FFT timingparameter, a means to determine, based at least in part on a channelimpulse response estimate associated with a first received symbol, anFFT timing adjustment parameter, and a means to adjust the initial FFTtiming based at least in part on the FFT timing adjustment parameter soas to provide an adjusted FFT timing.

In another aspect, the disclosure relates to a computer program product.The computer program product may include a computer-readable mediumhaving codes for causing a processor to implement or initiateimplementation of one or more of the stages of setting an initial timingof the FFT module based on an initial FFT timing parameter, determining,based at least in part on a channel impulse response estimate associatedwith a first received symbol, an FFT timing adjustment parameter, andadjusting the initial FFT timing based at least in part on the FFTtiming adjustment parameter so as to provide an adjusted FFT timing.

In another aspect, the disclosure relates to an apparatus forcontrolling FFT timing in a receiver. The apparatus may include one orboth of a signal characterization module disposed to generate a channelresponse estimate based on a first received symbol signal, and a timingcontrol module disposed to generate an FFT timing adjustment parameterbased at least in part on the channel response estimate.

The signal characterization module may include one or both of an IFFTmodule coupled to the output of a demodulator FFT module, and a channelimpulse response estimator module coupled to an output of the IFFTmodule. The channel impulse response estimator may be configured togenerate the channel impulse response estimate based at least in part onthe output of the IFFT module. The timing control module may include oneor both of a variable rate interpolator, and a fine symbol timing moduledisposed to generate, based at least in part on the channel impulseresponse estimate, the FFT timing adjustment signal as an error signal.The error signal may be generated so as to adjust the sample rate of thevariable rate interpolator responsive to the channel impulse responseestimate.

The apparatus may further include an FFT module. The FFT module may beconfigured to receive an output of the variable rate interpolator, andgenerate an output signal approximating a transmitted symbolcorresponding to the received symbol.

The fine symbol timing module may include one or more of an absolutevalue module configured to generate an absolute value signal based onthe channel impulse response estimate, a peak locator module configuredto detect one or more peak values in the absolute value signal, and anerror calculation module configured to generate the FFT timingadjustment signal.

The signal characterization module may further include a clock errormodule disposed to generate a clock error signal. The timing controlmodule may include one or both of a combiner module and a variable rateinterpolator module. The combiner module may be configured to combinethe FFT timing adjustment signal and the clock error signal so as togenerate, as an output, a combined error signal. The output may becoupled to an input of the variable rate interpolator.

In another aspect, the disclosure relates to a method for adjusting avariable rate interpolator in an OFDM receiver. The method may includeone or more of generating a channel impulse response estimate for areceived OFDM signal, determining the location of one or more peakvalues in the channel impulse response estimate, generating a timingerror signal based at least in part on the one or more peak values, andadjusting the variable rate interpolator responsive to the timing errorsignal.

The estimating the location of one or more peak values in the channelimpulse response may include one or more of the stages of selecting athreshold value, determining one or more pairs of threshold crossinglocations of the channel impulse response estimate, and generating thelocation estimates of the one or more peak values based on ones ofcorresponding one or more pairs of threshold crossing locations. Theones of the location estimates of the one or more peak values may begenerated as the average of ones of the corresponding threshold crossinglocations.

The generating a timing error adjustment signal may include one or moreof the stages of comparing the one or more peaks to a set of one or morereference values, selecting ones of the estimates of the one or morepeaks that are within a predefined search distance WIN of correspondingones of the one or more reference values, and generating the timingerror adjustment signal as a function of the difference between theselected ones of the one or more peak values and the corresponding onesof the one or more reference position values. The function of thedifference between the selected ones of the one or more peak values andthe corresponding one of the one or more reference position may be thesum of the differences. The generating the channel response estimate forthe OFDM signal may be based on a pilot tone included in the OFDMsignal.

In another aspect, the disclosure relates to a communication apparatus.The communication apparatus may include a processor module configured toimplement or initiate implementation of one or more of the stages ofgenerating a channel impulse response estimate for a received OFDMsignal, determining a location of one or more peak values in the channelimpulse response estimate, generating a timing error signal based atleast in part on the one or more peak values, and adjusting the variablerate interpolator responsive to the timing error signal.

In another aspect, the disclosure relates to a communication apparatus.The communication apparatus may include one or more of a means togenerate a channel impulse response estimate for a received OFDM signal,a means to determine a location of one or more peak values in thechannel impulse response estimate, a means to generate a timing errorsignal based at least in part on the one or more peak values, and ameans to adjust the variable rate interpolator responsive to the timingerror signal.

In another aspect, the disclosure relates to a computer program product.The computer program product may include a computer-readable mediumhaving codes for causing a processor to implement or initiateimplementation of one or more stages of generating a channel impulseresponse estimate for a received OFDM signal, determining a location ofone or more peak values in the channel impulse response estimate,generating a timing error signal based at least in part on the one ormore peak values, and adjusting the variable rate interpolatorresponsive to the timing error signal.

In another aspect, the disclosure relates to an apparatus for adjustinga variable rate interpolator in an OFDM receiver. The apparatus mayinclude one or both of a channel impulse response estimator circuitdisposed to generate a channel impulse response estimate for a receivedOFDM signal, and a fine symbol timing (FST) circuit disposed to generatean error signal to be used at least in part to adjust the variable rateinterpolator, wherein the error signal is generated based on one or morepeaks of the channel impulse response estimate.

The channel impulse response estimator may include a circuit to generatethe channel impulse response estimate based on a pilot tone provided inan OFDM signal received by the OFDM receiver. The FST circuit mayinclude one or both of a peak locator circuit disposed to estimate theposition of one or more peaks in the channel impulse response estimate,and an error determination circuit disposed to determine a positionerror between the estimate of the one or more peaks and one or morereference values. The apparatus may further include an updating circuitconfigured to update and store the peak reference values, and providethe updated peak reference values to the error determination circuit.

The peak locator circuit may be configured to estimate the location ofthe one or more peaks in the channel impulse response estimate by one ormore of the stages of selecting a threshold value, determining one ormore pairs of threshold crossing locations of the channel impulseresponse estimate, and generating the peak location estimates of the oneor more peak values based on corresponding one or more pairs ofthreshold crossing locations. The ones of the peak location estimates ofthe one or more peak values may be generated as the average of ones ofthe corresponding threshold crossing locations.

The error signal may be generated by one or more stages of comparing theone or more peaks to a set of one or more reference position values,selecting ones of the estimates of the one or more peaks that are withina search distance of corresponding ones of the one or more referenceposition values, and generating the error signal as a function of thedifference between the selected ones of the one or more peak values andthe one or more reference values. The function of the difference betweenthe selected ones of the one or more peak values and the one or morereference values may be the sum of the differences.

Additional aspects of the present invention are further described belowin conjunction with the appended Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is more fully appreciated in connection with thefollowing detailed description taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 illustrates a typical OFDM communications system subject tomultipath interference;

FIG. 2 illustrates an embodiment of an OFDM communications systemconsistent with the present invention;

FIG. 3A illustrates an OFDM modulation architecture;

FIG. 3B illustrates an OFDM receiver demodulation architecture inaccordance with aspects of the present invention;

FIG. 4A illustrates an example of multipath signal reception in an OFDMreceiver system;

FIG. 4B illustrates examples of timing of FFT sampling for the signalsshown in FIG. 4A;

FIG. 5 illustrates embodiments of initial FFT timing determination in anOFDM receiver;

FIG. 6 is a high level illustration of a circuit implementing anembodiment of the present invention for FFT timing adjustment signalgeneration;

FIG. 7 illustrates additional details of an embodiment of timingdetection and correction processing in accordance with aspects of thepresent invention;

FIG. 8A illustrates an example signal and associated processing signalparameters for peak determination in accordance with aspects of thepresent invention;

FIG. 8B illustrates an example signal and associated processing signalparameters for peak detection and tracking in accordance with aspects ofthe present invention;

FIG. 8C illustrates an example signal having multiple peaks, andassociated processing signal parameters for peak detection and trackingin accordance with aspects of the present invention;

FIG. 9 illustrates an embodiment of a process for fine timingdetermination in accordance with aspects of the present invention;

FIG. 10 illustrates an embodiment of a process for controlling FFTmodule timing, such as in a communication receiver; and

FIG. 11 illustrates an embodiment of a process for adjusting a variablerate interpolator, such as in a communication receiver.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

The present invention is directed generally to apparatus and methods forreceived signal timing detection, tracking, and error correction.Typical embodiments of the invention may be used in orthogonal frequencydivision multiplexing (OFDM) communications systems; however,embodiments of the present invention may also be used in othercommunications systems consistent with the features and functionalitydescribed herein.

For example, various embodiments of the invention may be used todetermine and track an optimal FFT sampling window for demodulating areceived OFDM signal and corresponding OFDM symbols that propagatethrough a time-varying channel based on controlling FFT timing. Theposition of optimal symbol sample timing may be determined based on thereceived OFDM signal, which may be subject to Doppler and multipathdistortion and therefore may include frequency shifts and multiplereflected signals. Once the position of optimal sample timing has beendetermined, the sampled OFDM signal may then be demodulated and thetransmitted symbol detected.

As described herein, in an exemplary embodiment, optimal sample timingmay be done by adjusting the sampling rate of received signal samplesprovided to an FFT block in an OFDM receiver. However, FFT timing (orother demodulation processing mechanisms) may alternately be adjusted byproviding a timing or triggering signal to the FFT or other demodulationblocks in some embodiments, and other timing adjustments based on thesignal processing mechanisms described herein may also be used in someimplementations.

In addition to determining optimal FFT timing, FFT timing or windowpositioning may be tracked over time to accommodate movement of thereceiver and/or multipath reflectors as well as addition or removal ofmultipath reflectors from the signal path.

Attention is now directed to FIG. 1, which illustrates an examplecommunication system 100 subject to multipath interference on whichembodiments of the present invention may be implemented. System 100includes a transmitter 120 and receiver 110, as well as one or moremultipath reflectors 130 a-n. Reflectors 130 are shown in FIG. 1 asbuildings; however, many other types of structures or objects maylikewise constitute multipath reflectors in various communicationssystems on which embodiments of the present invention may beimplemented. These may include, for example, building walls, metallic orother reflective structures, vehicles, or other types ofelectromagnetically reflective objects or structures.

Receiver 110 may be in a fixed position relative to the reflectors 130a-n, or may move along a path 115 relative to the reflectors, as shownin FIG. 1. In addition, the reflectors 130 a-n may also move (not shown)and/or reflectors may be added to, or removed from, system 100.

In system 100, the transmit signal provided by transmitter 120 typicallypropagate along multiple paths to receiver 110. FIG. 1 shows a directsignal path, labeled as Path 0, along with n reflected paths (Paths 1,2, . . . n). Signals transmitted along the various paths may be timeshifted relative to each other based on their different path lengths, aswell as attenuated and/or frequency (Doppler) shifted. Consequently, thesignal received at receiver 110 may include two or more superimposedsignals from the various paths. Example signals are shown in FIG. 3A andfurther described subsequently herein.

As noted previously, an advantage of OFDM systems is their ability totolerate multipath signals such as those from the multiple signal pathsshown in FIG. 1. Because the incoming data signal is divided intomultiple sub-channels, the relatively low symbol rate used in thesub-channels allows for adding a guard interval between symbols, makingit possible to compensate for time-spreading and intersymbolinterference (ISI). In many OFDM systems, a symbol is transmitted alongwith a cyclic prefix (CP) as part of the guard interval, which allowsthe receiver to better tolerate multipath and hence ISI. The CPtypically consists of a replicated portion of the end of the symbolappended to the beginning of the symbol, which enables the receiver tointegrate the received signal over an integer number of sinusoidalcycles. FIG. 3A illustrates an example of combined received symbols andassociated CPs for the various signal paths as shown in FIG. 1.

One implementation advantage of OFDM systems in the ability toefficiently modulate and demodulate an OFDM signal using inverse FastFourier Transforms (IFFTs) and Fast Fourier Transforms (FFTs),respectively. This avoids the need for tuned sub-channel receiverfilters and associated circuitry as are commonly used in convention FDMsystems. At the receiver, an FFT module is used to demodulate thereceived signal, providing potential advantages from the low cost andwide availability of FFT processing hardware and software. However,timing of the FFT processing in the receiver is important to receiverperformance. In the presence of multipath, there is typically an optimaltiming window or position that maximizes received signal energy andwhich can be set by adjusting the FFT timing. The optimal FFT location,or “window,” may shift forward or backward due to a variety of factorsincluding differences in sample timing between transmitter and receiveras well as changes in the channel through which the transmitted signalis sent.

For example, if the clock reference of the receiver is offset from thatof the transmitter, the position of the received symbols in sample spacewill slip forward or backward, all other factors being equal. If thereceiver 110 moves along path 115, the received signals from Paths 0through N may shift, attenuate, split into multiple paths and/or othernew paths may emerge as the wireless channel changes. Consequently, onepotential advantage of the present invention is to allow a receiver totrack optimal FFT timing regardless of changes in the wireless channel.In an exemplary embodiment, this may be done by adjusting the clockingor sample rate of the received signals by resampling them to an adjustedsample rate. However, in other embodiments, similar or equivalentadjustments, such as providing an FFT trigger signal or other FFT timingadjustment mechanism may also be used.

Attention is now directed to FIG. 2, which illustrates a high level viewof a typical OFDM communications system 200 on which embodiments of thepresent invention may be implemented. Communications system 200 includesa transmit sub-section 220, which may correspond with transmitter 120 asshown in FIG. 1, and a receive sub-system 210, which may correspond withreceiver 110 of FIG. 1. Transmit sub-system 220 includes a modulationmodule 212 configured to receive either a single data channel ormultiple data channels. The sub-system 220 demultiplexes (in the case ofa single incoming channel) the digital data into multiple transmitsub-channel signals ST1-STn, or, in the case of multiple input channels,distributes the multiple channels to corresponding transmit sub-channelssignals ST1-STn.

An IFFT module 213 then generates I and Q modulated outputs byimplementing an IFFT on the transmit sub-channel signals ST1-STn. Animplementation of one embodiment of a process effected by the modulationmodule 212 is described in further detail below and shown in FIG. 3A.The modulated signal may then be upconverted to an intermediatefrequency (IF) in mixer module 214, and then further upconverted to atransmit frequency in transmit module 216. A transmitted signal (TXSignal) is sent from a transmit antenna (TX Ant.), where it may then besubject to variable attenuation, noise, Doppler effects, and/ormultipath as is shown in FIG. 1, resulting in combined receive signalssuch as are shown in FIG. 3A.

The receive sub-system 210 receives the transmitted signal, noise, andmay also receive one or multipath signals such as those shown in Paths1-n in FIG. 1. The composite received signal is subject to channeldegradation and noise, and may also be affected by Doppler effects orother distortions. Consequently, the received signal must be processedin the receive sub-system 232 to extract an approximation of thetransmit signal. In the example OFDM receiver shown in FIG. 2, adownconverter stage 238 may be used in the receiver to convert thereceived signal to an IF for processing (or, in some implementations thereceived signal may be directly converted to baseband). A baseband I/Qmodule 236 then converts the IF signal to baseband I/Q signals, whichmay then be A/D converted to digital I and Q signals (shown as I1 and Q1in FIG. 2), which are then provided to demodulation module 232.

Finally, in accordance with aspects of the present invention,demodulation module 232 performs timing adjustment and other processingto the received signal and generates adjusted I and Q signals in module234 as further described subsequently herein. The adjusted I and Qsignals may be processed in an FFT module 235 to extract the varioussub-carrier channel signals into demodulated output data signals.

The timing of the FFT module may be controlled in timing detection andprocessing module 234, which performs timing adjustment based in part oncharacterization of the received signal as provided as output from theFFT module 235. Output signals are provided from module 234 as signalsSR1-SRn. These signals may then be converted to the time domain toreplicate the original sub-channel signals. The received sub-channelsignals may then be provided as a plurality of digital outputs (ifmultiple channels were provided as inputs) or re-multiplexed inmultiplexer 239 to approximate the original single channel datastreamprovided to transmit module 212. It is noted that this is a simplifiedexample of an OFDM transmitter and receiver architecture on whichembodiments of the present invention may be implemented, and that othercircuit elements as are known in the art have been omitted for thepurpose of clarity.

Attention is now directed to FIG. 3A which illustrates additionaldetails of one implementation of the modulation module 212. As shown inFIG. 3A, input data is divided into multiple sub-channels, with each ofthe subchannel symbols mapped to a corresponding point in the selectedsignal space. For example, in the two-dimensional signal space shown forST1 in FIG. 3A, a particular signal point S1 within the selected signalconstellation is transformed at frequency f1 in the IFFT module into Iand Q data. All of the corresponding sub-channels ST2-STn aresimultaneously transformed at frequencies f2-fn in the IFFT module andcombined so as to generate the OFDM symbol having a symbol durationTsym. This OFDM signal may then be upconverted to the transmit frequencyand sent to a corresponding receiver.

An example of an embodiment of a corresponding demodulation module 232in accordance with aspects of the present invention is shown in FIG. 3B.Module 232 includes a demodulation subsystem 234 which may include anFFT subsystem 235 configured to regenerate the subchannels as SR1-SRnusing an FFT and inverse process to that shown in FIG. 3A. In addition,module 234 may include a signal characterization module 240, which isconfigured to determine signal characteristics of the FFT output and/orprovide output data as shown in FIG. 3B, as well as a timing control andtracking module 250, which is configured to set and adjust FFT timingand/or store information about the FFT timing parameters so as to trackand adjust FFT timing over time. As noted previously, since the receivedsignal may include noise and multipath signals and/or other distortion,timing of the FFT is typically important to receiver performance.

This problem is further illustrated in FIG. 4A, which shows an exampledirect signal (on Path 0 as shown in FIG. 1), combined with additionalmultipath signals on Paths 1-n. At the receiver module 232, the varioussignals shown in FIG. 4A are effectively added in the received signal tocomprise a combined or composite signal with overlapping symbols fromthe various paths. Determination of FFT timing, in view of the combinedsignal characteristics, becomes an important concern to optimizereceiver performance.

Referring further to FIG. 4A, transmitted symbols S0, S1 and S2 areshown, with a CP appended in a guard band at the beginning of eachsymbol. The effect of multipath is to superimpose multipath signals fromPaths 1-n onto the direct signal received from Path 1, which affects theoptimal timing for performing the FFTs in subsystem 235. Consequently,there is an optimal time window in which to perform the FFT operation onthe received symbols, as further described below.

Attention is now directed to FIG. 4B, which illustrates various possibletiming windows for performing the FFT, as well as an optimal timingwindow in accordance with aspects of the present invention. As shown inFIG. 4B, the window duration for performing the FFT corresponds to theduration of a transmitted symbol, Tsym, and this window can be timeshifted relative to the received symbols S0, S1, . . . SN. As a firstexample, starting the FFT at time T1 as shown in FIG. 4B will result inan early window since initiating the FFT at T1, when symbol S1 begins,will cause the FFT to include energy from multipath signals from theprevious Symbol (S0) received from Paths 1-n and will exclude somesignal energy from symbol S1 from the multipath signals.

Alternately, starting the FFT processing of symbol S1 at time T3 willresult in a late window since this timing will include signal energyfrom the next symbol, S2 from the direct path, Path 0 and likewiseexclude some signal energy from the direct signal. In either scenario,signal energy from adjacent symbols will be included in the FFTprocessing while energy from the transmitted symbol will be excluded,thereby decreasing symbol detection performance.

Consequently, there is an optimal FFT start time, T2 as shown in FIG.4B, such that the FFT window includes a minimal amount of energy fromadjacent symbols. Alternately, the FFT window may be placed so that itincludes the channel peak value with a predetermining timing margin (forexample, +/−n,m samples). In this approach, if the channel peak islocated on the Pth sample within one OFDM symbol period, the startingposition of the FFT window could be set at the P-nth sample. Moreover,as noted previously, this optimal timing window may move over time asthe receiver position changes and/or as multipath reflectors are addedor removed from the communication channel or are moved relative to thetransmitter or receiver (or due to movement of the receiver relative tothe transmitter). Accordingly, one potential advantage of the presentinvention is to allow a receiver to determine and track the optimaltiming of the FFT regardless of changes in the wireless channel. Thisoptimal timing may be referred to herein as an optimal FFT timing oroptimal FFT timing window.

Attention is now directed to FIG. 5, which illustrates details ofimplementations for determining initial FFT sample window timing basedon an initial FFT timing parameter. As shown in FIG. 5, initial FFTwindow placement may be done by cross-correlation or auto-correlationbased methods, as well as by other methods known or developed in theart. For example, In a cross-correlation implementation, the repetitiousstructure of a signal such as the CP or another signal that is repeatedin the time domain (such as is used in WiMax, CMMB) may be used. Anexample of this approach is shown in 500A, which illustrates shifting aCP relative to the received signal and then determining relative FFTtiming based on the peak correlation signal. Once this peak isdetermined, the FFT timing can be set based on an initial FFT timingparameter, such as a relative offset from the determined peak. This maybe done by advancing or retarding FFT timing based on a number ofsamples corresponding to the relative timing difference between the CPpeak and the start of the next symbol.

Similarly, in an auto-correlation based implementation as shown in 500B,a known training signal or training symbols may be used. Initial FFTwindow positioning may be determined by an initial FFT timing parameter,which may be, for example, an offset distance between the correlationpeak position and a predetermined value. Tracking may be done by variousmethods, but typically initial FFT window timing should be determined ina way so as to include significant peaks. Examples of approaches forinitial timing determination are described in, for example, U.S. Pat.Nos. 6,421,401, 6,650,617 as well as United States Patent Publication2006/0233097, the contents of which are hereby incorporated by referenceherein.

Attention is now directed to FIG. 6, which is a high level illustrationof an embodiment of aspects of the present invention. In an exemplaryembodiment, FFT timing is adjusted by adjusting the sample rate of thereceived signal so that the FFT start time can be triggered according toa known or reference sample count. As noted previously, in someembodiments other methods for controlling FFT timing may alternately beused.

Returning to FIG. 6, in an exemplary embodiment a variable rateinterpolator 610 may be used to adjust the sample timing in response toan FFT timing adjustment parameter, which in the illustrated example isan error or control signal provided to the interpolator. By adjustingthe sample timing, the FFT timing is correspondingly adjusted, assumingthe FFT is configured to operate periodically based on the number ofreceived samples. At a high level, the embodiment of FIG. 6 includes theFFT module 235 as well as exemplary components of modules 240 and 250 asshown in FIG. 2. Module 240 is configured to receive the FFT outputs andprovide output data signals as well as signals characterizing thereceived OFDM signal and associated channel. Module 250 is configured toadjust the FFT timing in response to the output of module 240.

More specifically, baseband I and Q signal samples (IQ1[n],corresponding to I1 and Q1 as shown in FIG. 2) from a received OFDMsignal may be provided from a communication receiver module (forexample, module 236 and A/D converters as shown in FIG. 2A) to variablerate interpolator (VRI) 610. Composite error signal eT[n] is provided toVRI 610 to correct sample timing, which will corresponding adjust theFFT timing in FFT module 235.

VRI 610 provides, as an output, adjusted I and Q samples signal IQ2[n]mwhich are set at a sample rate determined by VRI 610 in response toinput error signal eT[n]. The output sample rate may be set to a highervalue than the input rate if the polarity of eT[n] is positive, orcorrespondingly to a lower value if the polarity of eT[n] is negative(alternately, other error signal to sample rate adjustment mappings maybe used).

The output of FFT module 235 may then be used to generate the digitaloutput data as shown in FIG. 2. In addition, the output of the FFTmodule 235 may be further processed so as to generate parameterscharacterizing the OFDM signal and/or the communications channel. Inparticular, in an exemplary embodiment, the output of FFT module 235 isprovided to an IFFT module 640 which converts the FFT output signal tothe time domain for further processing in Channel Impulse ResponseEstimation Module (CIRE) 615. The output of the FFT module may also beprovided to a Clock Error Determination Module (CERR) 625.

CERR 625 and CIRE 620 estimate the clock error and channel impulseresponse, based on IQ2[n], respectively. This may be done by forexample, using techniques such as are described in Pei-Yun Tsai, Hsin-YuKang and Tzi-Dar Chiueh, “Joint Weighted Least-Squares Estimation ofCarrier-Frequency Offset and Timing Offset for OFDM Systems OverMultipath Fading Channels”, IEEE Trans. On Vehicular Tech. Vol. 54, No1,January 2005 for CERR determination, and Van de Beek, J.-J., Edfors, O.S., Sandell, M., Wilson, S. K., and Börjesson, O. P., “On channelestimation in OFDM systems,” 45th IEEE Vehicular Technology Conference,Chicago, Ill., vol. 2, pp. 815-819, July 1995 for CIRE determination, orusing other techniques as are known or developed in the art.

For example, CERR 625 may convert IQ2[n] to the frequency domain toextract the clock error, as is commonly done in OFDM systems; while CIREmay use pilot symbols or pilot tones as are commonly provided in OFDMsystems to estimate the channel impulse response. Other channel impulseresponse estimation methods and apparatus as are known or developed inthe art may also be used.

The output of CIRE 620, which includes data representing the estimatedchannel impulse response, may then be sent to Fine Symbol Timing (FST)Module 620 which further estimates the symbol timing error forhigher-resolution correction of the sampling error in IQ2 and generatesan FFT timing adjustment parameter to adjust the FFT timing. Additionaldetails of an embodiment of FST 620 are shown in FIG. 7 and describedbelow.

In addition, as shown in FIG. 6, the error signals provided by CERR 625and FST 620 may be selectively combined in combiner module 630 so as toprovide a composite error correction signal eT[n] to VRI 610 tocompensate for both clock error and symbol timing error. In an exemplaryembodiment, this may be done as follows. Initially, the FST module 620or FST module 620 output is disabled and initial FFT timing is set byCERR 625 module. Once determined, this value does not typically changerapidly, and therefore it may be set to a fixed value which may beeither maintained for a relatively long duration, updated periodically,or updated asynchronously based on a particular signal or channelcondition. In general, the output eCLK[n] from module CERR 625 will befixed for a long duration relative to the update rate of module FST 620.Accordingly, in an exemplary embodiment, once the output of CERR 625 isinitially set, it will be maintained at a fixed value while the errorsignal eFST[n] from module 620 is repeatedly updated. The two signalsare combined in module 630, with the composite error signal eT[n] thenprovided to VRI 610 to adjust the output sample rate responsive to theerror signal. In some embodiments CERR 625 and FST 620 may becommunicatively coupled as shown in FIG. 6 to coordinate communicationbetween the modules to adjust their error signals synchronously.

Attention is now directed to FIG. 7, which illustrates one embodiment ofFST module 620. At a high level, FST Module 620 is configured to receivea channel impulse response estimate and then generate an FFT timingadjustment parameter, in the form of an error signal for adjusting theFFT timing in conjunction with VRI 620, based at least in part on thechannel impulse response estimate. In alternate embodiments (not shown),FST module 620 may be configured to generate the FFT timing adjustmentparameter as an FFT trigger signal or other FFT control signal, based onthe processing described herein with respect to FST 620, rather thanproviding an error signal to adjust VRI 620.

As shown in FIG. 7, in an exemplary embodiment, the channel impulseresponse estimator output CIR[n] may first be provided to an absolutevalue module 710, which is configured to generate a signal correspondingto the absolute value of CIR[n] representing the magnitude of thechannel impulse response estimate. The absolute value of the channelimpulse response CIR[n] may then filtered by Filter module F1 715, whichis typically a lowpass filter, and the resulting filtered signal s[n] issent to peak locator module PL1 720, which generates peak valuelocations P_(j)[n] associated with the channel response estimate.

Example signals that may be generated by FST 620 are illustrated inFIGS. 8A-8C, with relevant signal parameters shown. Error determinationmodule 725 generates an error signal to facilitate adjustment of FFTtiming (as noted previously, this may also be a trigger signal or othersignaling mechanism to control FFT timing in alternate embodiments). Apeak update and tracking module 728 may be coupled to the errordetermination module 725. Peak update modulate 728 is configured toreceive and store initial values of the estimated peaks (as detectedduring an initial processing cycle an/or on a later re-calibrationcycle) as peak reference values (denoted herein as Xref values), as wellas receive and store updates of the peak reference values. These valuesmay then be provided to the error calculation module 725 for subsequentpeak location tracking.

Further addressing the error calculation process, in one embodiment,error signal generation as may be performed in error determinationmodule 725 is further described below in conjunction with FIG. 9.

FIG. 9 illustrates an embodiment of a process 900 associated with peakposition error determination and tracking as may be performed in modules725 and 728. In summary, at the start of a received symbol the processdetermines an estimation of the location of one or more peaks above athreshold value in the channel impulse response estimate, and stores thelocation of the peaks as peak reference values. Then during processingof subsequent received symbols, the peak location values are tracked andan error signal generated based on changes in the position of the peaksrelative to the peak reference values.

More specifically, process 900 may begin at stage 910 when a new symbolS[n] is received. At stage 912 a threshold value, TH0, is set. Thisvalue may be based on parameters such as received signal strength as maybe provided by a received signal strength indicator (RSSI) (not shown)or other signal strength detection apparatus included in receiver module232, and/or based on other parameters such as noise level, distortionfloor or other signal and/or noise parameters that are likewise providedby corresponding modules (not shown). The threshold value TH0 may becontinuously monitored and updated in some embodiments (such as uponreceipt of each new symbol) or may be set for a give time interval andthen periodically or asynchronously updated.

Once the threshold value TH0 is set for a particular received symbol orsymbols, crossings of the channel impulse response estimate above andbelow the threshold are determined, and location estimates of peaksassociated with corresponding pairs of threshold crossings are thendetermined at stage 915. An example of aspects of this process is shownin further detail in FIG. 8A, with the positive slope threshold crossingdenoted as sub-e (i.e. the jth positive slope threshold position Xej)and the negative slope threshold denoted as sub-1 (i.e., Xlj).

For each pair Xej and Xlj, an estimate of the location of thecorresponding peak, Pj[n] (representing the jth peak value of the nthsymbol), is determined This may be done by, for example, averaging theposition values of the associated threshold crossings Xej and Xlj todetermine the estimated peak location. Other methods may also be used,such as by performing a weighted average of samples between Xej and Xlj,or by other methods.

Once all of the peak locations are determined for a particular symbol,S[n] (i.e., the j peak values for symbol n) an error signal may begenerated based on a function of the determined peak locations. Intypical embodiments this will comprise a two-step search process, withan initial search performed at the start of the detection process to setreference values based on the initial peak locations, such as duringreceipt of a test or first symbol) with updated searches and associatedtracking and adjustment performed on successive symbols.

This two-step process may be done as shown in process 900 by performinga test for the initial search at stage 925. If the search in an initialsearch, the j peak location values (i.e., Pj[0]s) determined initiallymay be stored as peak reference values Xrefq[n] (with change in indexfrom j to q, where q equals the number of initial peaks determined) atstage 935 for use in subsequent iterations, and the initial error signaleFST[n] initialized to a reference value which may be set to a valuebased on the initial Xref[n] values or set to a zero or otherinitialization value. The Xref values may be updated and stored bymodule 728 as shown in FIG. 7.

Upon receipt of a subsequent symbol, the process stages 910, 912, 915and 920 and 925 may be repeated to update the peak location estimatesfor the subsequent symbol. At stage 925, execution may continue to stage930 where changes in peak locations may be detected and updating of thepeak reference values may be performed. Details of this aspect ofprocess 900 are shown in FIG. 8B with reference to a symbol receivedsubsequent to the symbol shown in FIG. 8A. Specifically, on the secondand subsequent iteration(s) of process 900 at stage 930, updated peaklocations within a search range “WIN” of the initial peak referencelocations are identified (As noted below, on each iteration thereference location values may change, and therefore the search range WINmay be updated if the reference location values change). The value ofWIN may be adjusted dynamically in some embodiments based on signal orchannel characteristics, however, in an exemplary embodiment the valueof WIN is a pre-determined fixed value. For each of the determined peakvalues that fall within the range WIN, an offset distance value isdetermined. For example, as shown in FIG. 8B, the offset value may bedetermined as the difference between the reference value (Xrefj[n−1])and an associated peak value Pj[n]. FIG. 8C illustrates a correspondingexample where there are two peaks in the response, and two correspondingreference values.

Once the offset is determined for all of the peaks with the range WIN ofthe references, a timing error adjustment signal eFST[n] may begenerated. This signal may be based on a function of the differencesbetween the peak values and the references, such as by taking the sum ofthe differences between the references and peak values (i.e.,eFST=sum(Xrefk[n−1]-Pq[n]) over all q (where q=number of determinedpeaks within the range WIN). Alternately, other metrics may be used. Forexample, in some embodiments a non-linear metric, such as the cube ofthe differences may be used.

Once the timing error adjustment signal eFST[n] is determined, it may besent to a filter (e.g., accumulator) such as loop filter F2 as shown inFIG. 7. As further shown in FIG. 7 and described previously herein, thefiltered error signal may be combined with any error signals generatedby CERR 625 or other error signal generators (not shown) to be providedto VRI, where it may be used to advance or retard the sample timing andcorrespondingly the FFT timing.

As noted previously, in an exemplary embodiment, FFT window placement isdone after the resampling process performed in VRI 610. The FFTprocessing is typically done by counting the number of received samples.For example, suppose the CP and OFDM symbol length are M and N,respectively, and the FFT window starts right after the CP. FFT windowplacement may be done by counting the number of input samples, whichplaces the FFT window from M+1 to M+N samples every time it receives M+Nsamples. This is not changed even if VRI 610 changes sampling rate.However, if VRI 610 increases the sampling frequency, the FFT windowwill advance in time, and if VRI 610 decreases the sampling frequency,the FFT window will be retarded in time. Thus, changing the samplingfrequency in VRI provides an efficient method for adjusting the FFTwindow timing.

New peaks may appear or disappear in the channel impulse response. Thismay be accommodated by a variety of approaches, including adjusting thereference location whenever there is no peak detected within +/−WINaround the reference locations. In this case, the initialization processmay be repeated and subsequently followed by the tracking processdescribed herein. Alternately, if there is at least one peak within+/−WIN around any reference location, the references need not beupdated.

FIG. 10 illustrates details of a process 1000 that may be used to, forexample, control FFT module timing, such as in an OFDM receiver orreceiving apparatus. At stage 1010, an initial timing of the FFT modulebased may be set, which may be based on, for example, an initial FFTtiming parameter. At stage 1020, a stage of determining, based at leastin part on a channel impulse response estimate associated with a firstreceived symbol, an FFT timing adjustment parameter may be performed. Atstage 1030, the initial FFT timing may be adjusted based at least inpart on the FFT timing adjustment parameter, so as to provide anadjusted FFT timing.

The initial FFT timing may be adjusted in process 1000 by, for example,providing a resampled FFT input signal based on the received signal. Thesample rate of the resampled signal may be set based at least in part onthe FFT timing adjustment parameter. The FFT timing adjustment parametermay be an error signal. A variable rate interpolator may be configuredto generate the resampled FFT signal at a sample rate that may be basedat least in part on the FFT timing adjustment parameter. The FFT timingadjustment parameter may be an error signal.

The stage 1020 of determining an FFT timing adjustment parameter mayinclude, for example, one or more of the steps of receiving the channelimpulse response estimate, determining one or more peaks in the channelimpulse response estimate, comparing the one or more peaks to a set ofone or more reference values, and generating the FFT timing adjustmentparameter based at least in part on said comparing.

The one or more reference values may be generated by a process that mayinclude, for example, one or more stages of setting a threshold value,determining one or more pairs of threshold crossing points, generatingthe one or more peaks based on the one or more pairs of thresholdcrossing points, and storing the one or more peaks as the referencevalues. The stage of determining one or more peaks may include one ormore of the stages of receiving the channel impulse response estimate,determining one or more sets of threshold crossing points, andgenerating the one or more peaks based on the one or more sets ofthreshold crossing points. The ones of said one or more peaks may begenerated as the average of corresponding ones of the threshold crossingpoints of the one or more sets of threshold crossing points. A stage ofgenerating the FFT timing adjustment parameter based at least in part onsaid comparing may include one or both of the stages of determining asubset of peaks within said one or more peaks, said subset consisting ofpeaks within a distance WIN of ones of the reference points, andgenerating the FFT timing adjustment parameter as a function of thedifferences between ones of the subset of peaks and corresponding onesof the reference points. The process 1000 may further include generatingan updated FFT timing adjustment responsive to receipt of a subsequentsymbol at the receiver

In another aspect, the disclosure relates to a communication apparatusconfigured to embody the process 1000. The communication apparatus mayinclude, for example, a processor module configured to: perform one ormore of the stages of setting an initial timing of an FFT module basedon an initial FFT timing parameter; determine, based at least in part ona channel impulse response estimate associated with a first receivedsymbol, an FFT timing adjustment parameter; and adjust the initial FFTtiming based at least in part on the FFT timing adjustment parameter soas to provide an adjusted FFT timing.

In one embodiment, the communications apparatus may include, forexample, one or more of a means to set an initial timing of an FFTmodule based on an initial FFT timing parameter, a means to determine,based at least in part on a channel impulse response estimate associatedwith a first received symbol, an FFT timing adjustment parameter, and ameans to adjust the initial FFT timing based at least in part on the FFTtiming adjustment parameter so as to provide an adjusted FFT timing.

Alternately, or in addition, process 1000 may be embodied in tangiblemedium, such as a computer program product. The computer program productmay include a computer-readable medium having codes for causing aprocessor to implement or initiate implementation of, for example, oneor more of the stages of process 1000, such as setting an initial timingof the FFT module based on an initial FFT timing parameter; determining,based at least in part on a channel impulse response estimate associatedwith a first received symbol, an FFT timing adjustment parameter; andadjusting the initial FFT timing based at least in part on the FFTtiming adjustment parameter so as to provide an adjusted FFT timing.

The process 1000 may be embodied in, for example, an apparatus forcontrolling FFT timing in a receiver. The apparatus may include one orboth of a signal characterization module disposed to generate a channelresponse estimate based on a first received symbol signal, and a timingcontrol module disposed to generate an FFT timing adjustment parameterbased at least in part on the channel response estimate. The signalcharacterization module may include one or both of an IFFT modulecoupled to the output of a demodulator FFT module, and a channel impulseresponse estimator module coupled to an output of the IFFT module. Thechannel impulse response estimator may be configured to generate thechannel impulse response estimate based at least in part on the outputof the IFFT module. The timing control module may include one or both ofa variable rate interpolator, and a fine symbol timing module disposedto generate, based at least in part on the channel impulse responseestimate, the FFT timing adjustment signal as an error signal. The errorsignal may be generated so as to adjust the sample rate of the variablerate interpolator responsive to the channel impulse response estimate.

The apparatus may further include, for example, an FFT module. The FFTmodule may be configured to receive an output of the variable rateinterpolator, and generate an output signal approximating a transmittedsymbol corresponding to the received symbol. The fine symbol timingmodule may include one or more of an absolute value module configured togenerate an absolute value signal based on the channel impulse responseestimate, a peak locator module configured to detect one or more peakvalues in the absolute value signal, and an error calculation moduleconfigured to generate the FFT timing adjustment signal.

The signal characterization module may further include, for example, aclock error module disposed to generate a clock error signal. The timingcontrol module may include one or both of a combiner module and avariable rate interpolator module. The combiner module may be configuredto combine the FFT timing adjustment signal and the clock error signalso as to generate, as an output, a combined error signal. The output maybe coupled to an input of the variable rate interpolator.

FIG. 11 illustrates details of a process 1100 that may be used, forexample, to adjust a variable rate interpolator, such as in acommunication receiver such as an OFDM receiver. At stage 1110, achannel impulse response estimate for a received OFDM signal may begenerated. At stage 1120, the location of one or more peak values in thechannel impulse response estimate may then be detected or determined. Atstage 1130, a timing error signal may be generated. The timing errorsignal may be based at least in part on the one or more peak values. Atstage 1140, the variable rate interpolator may be adjusted in responseto the timing error signal.

The stage 1120 of estimating the location of one or more peak values inthe channel impulse response may include, for example, one or more ofthe stages of: selecting a threshold value; determining one or morepairs of threshold crossing locations of the channel impulse responseestimate; and generating the location estimates of the one or more peakvalues based on ones of corresponding one or more pairs of thresholdcrossing locations. The ones of the location estimates of the one ormore peak values may be generated as the average of ones of thecorresponding threshold crossing locations.

The generating a timing error adjustment signal may include, forexample, one or more of the stages of: comparing the one or more peaksto a set of one or more reference values; selecting ones of theestimates of the one or more peaks that are within a predefined searchdistance WIN of corresponding ones of the one or more reference values;and generating the timing error adjustment signal as a function of thedifference between the selected ones of the one or more peak values andthe corresponding ones of the one or more reference position values. Thefunction of the difference between the selected ones of the one or morepeak values and the corresponding one of the one or more referenceposition may be the sum of the differences. The generating the channelresponse estimate for the OFDM signal may be based on a pilot toneincluded in the OFDM signal.

The process 1100 may be embodied in, for example, a communicationapparatus. The communication apparatus may include, for example, aprocessor module configured to implement or initiate implementation ofone or more of the stages of: generating a channel impulse responseestimate for a received OFDM signal; determining a location of one ormore peak values in the channel impulse response estimate; generating atiming error signal based at least in part on the one or more peakvalues; and adjusting the variable rate interpolator responsive to thetiming error signal.

The communication apparatus may include, for example, one or more of ameans to: generate a channel impulse response estimate for a receivedOFDM signal; a means to determine a location of one or more peak valuesin the channel impulse response estimate, a means to generate a timingerror signal based at least in part on the one or more peak values; anda means to adjust the variable rate interpolator responsive to thetiming error signal.

The process 1100 may be embodied in, for example, a computer programproduct. The computer program product may include a computer-readablemedium having codes for causing a processor to implement or initiateimplementation, for example, one or more of the stages of: generating achannel impulse response estimate for a received OFDM signal;determining a location of one or more peak values in the channel impulseresponse estimate; generating a timing error signal based at least inpart on the one or more peak values; and adjusting the variable rateinterpolator responsive to the timing error signal.

The process 1100 may be embodied in, for example, an apparatus foradjusting a variable rate interpolator, such as in an OFDM receiver. Theapparatus may include, for example, one or both of a channel impulseresponse estimator circuit disposed to generate a channel impulseresponse estimate for a received OFDM signal, and a fine symbol timing(FST) circuit disposed to generate an error signal to be used at leastin part to adjust the variable rate interpolator, wherein the errorsignal is generated based on one or more peaks of the channel impulseresponse estimate. The channel impulse response estimator may include acircuit to generate the channel impulse response estimate based on apilot tone provided in an OFDM signal received by the OFDM receiver. TheFST circuit may include one or both of a peak locator circuit disposedto estimate the position of one or more peaks in the channel impulseresponse estimate, and an error determination circuit disposed todetermine a position error between the estimate of the one or more peaksand one or more reference values. The apparatus may further include anupdating circuit configured to update and store the peak referencevalues, and provide the updated peak reference values to the errordetermination circuit.

The peak locator circuit may be configured to estimate the location ofthe one or more peaks in the channel impulse response estimate by, forexample, one or more of the stages of: selecting a threshold value;determining one or more pairs of threshold crossing locations of thechannel impulse response estimate; and generating the peak locationestimates of the one or more peak values based on corresponding one ormore pairs of threshold crossing locations. The ones of the peaklocation estimates of the one or more peak values may be generated asthe average of ones of the corresponding threshold crossing locations.

The error signal may be generated by one or more stages of, for example,comparing the one or more peaks to a set of one or more referenceposition values, selecting ones of the estimates of the one or morepeaks that are within a search distance of corresponding ones of the oneor more reference position values, and generating the error signal as afunction of the difference between the selected ones of the one or morepeak values and the one or more reference values. The function of thedifference between the selected ones of the one or more peak values andthe one or more reference values may be the sum of the differences.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any aspect and/or embodiment describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects and/or embodiments.

In some configurations, the apparatus for wireless communication mayinclude means for performing various functions as described herein. Inone aspect, the aforementioned means comprise a processor or processorsand associated memory in which embodiments reside, and which areconfigured to perform the functions recited by the aforementioned means.In another aspect, the aforementioned means may be other programmabledevices or other electronic or optical devices or other devices as areknown or developed in the art. In another aspect, the aforementionedmeans may be a module or any apparatus configured to perform thefunctions recited by the aforementioned means, which may be inassociation with the processes described herein, such as those describedwith respect to FIGS. 10 and 11.

As noted, various aspects of the present invention relate to one or moreprocesses such as are described and/or illustrated herein. Theseprocesses are typically implemented in one or more modules as aredescribed herein, and such modules may include computer software storedon a computer readable medium including instructions configured to beexecuted by one or more processors and/or associated process steps orstages. Alternately or in addition, embodiments of the processesdescribed herein may be embodied in hardware devices configured forimplementing analog or digital logic such as programmable logic devices,ASICs, DSPs, FPGAs, microprocessors, gate arrays or other electronicdevices.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration. In some implementations, processors may be processors,such as communication processors, specifically designed for implementingfunctionality in communication devices or other mobile or portabledevices.

The steps or stages of a method, process or algorithm described inconnection with the embodiments disclosed herein may be embodieddirectly in hardware, in a software module executed by a processor, orin a combination of the two and/or other components. A software modulemay reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROMmemory, registers, hard disk, a removable disk, a CD-ROM, or any otherform of storage medium known in the art. An exemplary storage medium iscoupled to the processor such that the processor can read informationfrom, and write information to, the storage medium. In the alternative,the storage medium may be integral to the processor. The processor andthe storage medium may reside in an ASIC. The ASIC may reside in a userterminal. In the alternative, the processor and the storage medium mayreside as discrete components in a user terminal.

It is noted that, while the processes described and illustrated hereinmay include particular steps or stages, it is apparent that otherprocesses including fewer, more, or different stages than thosedescribed and shown are also within the spirit and scope of the presentinvention. Accordingly, as noted previously, the processes andassociated modules shown herein are provided for purposes ofillustration, not limitation.

It is understood that the specific order or hierarchy of steps or stagesin the processes and methods disclosed are examples of exemplaryapproaches. Based upon design preferences, it is understood that thespecific order or hierarchy of steps in the processes may be rearrangedwhile remaining within the scope of the present disclosure. Theaccompanying method claims present elements of the various steps in asample order, and are not meant to be limited to the specific order orhierarchy presented.

Some embodiments of the present invention may include computer softwareand/or computer hardware/software combinations configured to implementone or more processes or functions associated with the present inventionsuch as those described herein. These embodiments may be in the form ofmodules implementing functionality in software and/or hardware softwarecombinations. Embodiments may also take the form of a computer storageproduct with a computer-readable medium having computer code thereon forperforming various computer-implemented operations, such as operationsrelated to functionality as describe herein. The media and computer codemay be those specially designed and constructed for the purposes of thepresent invention, or they may be of the kind well known and availableto those having skill in the computer software arts, or they may be acombination of both.

Examples of computer-readable media within the spirit and scope of thepresent invention include, but are not limited to: magnetic media suchas hard disks; optical media such as CD-ROMs, DVDs and holographicdevices; magneto-optical media; and hardware devices that are speciallyconfigured to store and execute program code, such as programmablemicrocontrollers, application-specific integrated circuits (“ASICs”),programmable logic devices (“PLDs”) and ROM and RAM devices. Examples ofcomputer code may include machine code, such as produced by a compileror other machine code generation mechanisms, scripting programs,PostScript programs, and/or other code or files containing higher-levelcode that are executed by a computer using an interpreter or other codeexecution mechanism.

Computer code may be comprised of one or more modules executing aparticular process or processes to provide useful results, and themodules may communicate with one another via means known or developed inthe art. For example, some embodiments of the invention may beimplemented using assembly language, Java, C, C#, C++, scriptinglanguages, and/or other programming languages and software developmenttools as are known or developed in the art. Other embodiments of theinvention may be implemented in hardwired circuitry in place of, or incombination with, machine-executable software instructions.

Those of skill will appreciate that the various illustrative logicalblocks, modules, circuits, and algorithm steps described in connectionwith the embodiments disclosed herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the disclosure.

The claims are not intended to be limited to the aspects shown herein,but are to be accorded the full scope consistent with the language ofthe claims, wherein reference to an element in the singular is notintended to mean “one and only one” unless specifically so stated, butrather “one or more.” Unless specifically stated otherwise, the term“some” refers to one or more. A phrase referring to “at least one of” alist of items refers to any combination of those items, including singlemembers. As an example, “at least one of: a, b, or c” is intended tocover: a; b; c; a and b; a and c; b and c; and a, b and c.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that specificdetails are not required in order to practice the invention. Thus, theforegoing descriptions of specific embodiments of the invention arepresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed; obviously, many modifications and variations are possible inview of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications. They thereby enable others skilled in the art tobest utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the following claims and their equivalents define thescope of the invention.

We claim:
 1. A method for controlling FFT module timing in a receiver,comprising: setting an initial timing of the FFT module based on aninitial FFT timing parameter; determining, based at least in part on achannel impulse response estimate associated with a first receivedsymbol, an FFT timing adjustment parameter; and adjusting the initialFFT timing based at least in part on the FFT timing adjustment parameterso as to provide an adjusted FFT timing, wherein the initial FFT timingis adjusted by providing a resampled FFT input signal based on thereceived signal and wherein the sample rate of the resampled signal isset based at least in part on the FFT timing adjustment parameter. 2.The method of claim 1, wherein the FFT timing adjustment parameter is anerror signal and wherein a variable rate interpolator is configured togenerate the resampled FFT signal at a sample rate based at least inpart on the FFT timing adjustment parameter.
 3. A method for controllingFFT module timing in a receiver, comprising: setting an initial timingof the FFT module based on an initial FFT timing parameter; determining,based at least in part on a channel impulse response estimate associatedwith a first received symbol, an FFT timing adjustment parameter; andadjusting the initial FFT timing based at least in part on the FFTtiming adjustment parameter so as to provide an adjusted FFT timing;wherein the FFT timing adjustment parameter is an error signal and saiddetermining an FFT timing adjustment parameter comprises: receiving thechannel impulse response estimate; determining one or more peaks in thechannel impulse response estimate; comparing the one or more peaks to aset of one or more reference values; and generating the FFT timingadjustment parameter based at least in part on said comparing.
 4. Themethod of claim 3, wherein the one or more reference values aregenerated by a process comprising: setting a threshold value;determining one or more pairs of threshold crossing points; generatingthe one or more peaks based on the one or more pairs of thresholdcrossing points; and storing the one or more peaks as the referencevalues.
 5. The method of claim 3, wherein the determining one or morepeaks comprises: receiving the channel impulse response estimate;determining one or more sets of threshold crossing points; andgenerating the one or more peaks based on the one or more sets ofthreshold crossing points.
 6. The method of claim 5, wherein ones ofsaid one or more peaks are generated as the average of correspondingones of the threshold crossing points of said one or more sets ofthreshold crossing points.
 7. The method of claim 3, wherein thegenerating the FFT timing adjustment parameter based at least in part onsaid comparing comprises: determining a subset of peaks within said oneor more peaks, said subset consisting of peaks within a distance WIN ofones of the reference points; and generating the FFT timing adjustmentparameter as a function of the differences between ones of the subset ofpeaks and corresponding ones of the reference points.
 8. The method ofclaim 3, further comprising generating an updated FFT timing adjustmentresponsive to receipt of a subsequent symbol at the receiver.
 9. Acommunication apparatus comprising a processor module configured to: setan initial timing of an FFT module based on an initial FFT timingparameter; determine, based at least in part on a channel impulseresponse estimate associated with a first received symbol, an FFT timingadjustment parameter; and adjust the initial FFT timing based at leastin part on the FFT timing adjustment parameter so as to provide anadjusted FFT timing, wherein the initial FFT timing adjustment comprisesproviding of a resampled FFT input signal based on the received signaland wherein the sample rate of the resampled signal is set based atleast in part on the FFT timing adjustment parameter.
 10. Acommunication apparatus, comprising: means to set an initial timing ofan FFT module based on an initial FFT timing parameter; means todetermine, based at least in part on a channel impulse response estimateassociated with a first received symbol, an FFT timing adjustmentparameter; and means to adjust the initial FFT timing based at least inpart on the FFT timing adjustment parameter so as to provide an adjustedFFT timing, wherein the initial FFT timing adjustment comprisesproviding a resampled FFT input signal based on the received signal andwherein the sample rate of the resampled signal is set based at least inpart on the FFT timing adjustment parameter.
 11. A computer programproduct comprising a computer-readable medium including codes forcausing a processor to: set an initial timing of the FFT module based onan initial FFT timing parameter; determine, based at least in part on achannel impulse response estimate associated with a first receivedsymbol, an FFT timing adjustment parameter; and adjust the initial FFTtiming based at least in part on the FFT timing adjustment parameter soas to provide an adjusted FFT timing, wherein the initial FFT timingadjustment comprises providing a resampled FFT input signal based on thereceived signal and wherein the sample rate of the resampled signal isset based at least in part on the FFT timing adjustment parameter. 12.An apparatus for controlling FFT timing in a receiver, comprising: asignal characterization module disposed to generate a channel responseestimate based on a first received symbol signal; and a timing controlmodule disposed to generate an FFT timing adjustment parameter based atleast in part on the channel response estimate, wherein the signalcharacterization module comprises: an IFFT module coupled to the outputof a demodulator FFT module; and a channel impulse response estimatormodule coupled to an output of the IFFT module, said channel impulseresponse estimator disposed to generate the channel impulse responseestimate based at least in part on the output of the IFFT module.
 13. Anapparatus for controlling FFT timing in a receiver, comprising: a signalcharacterization module disposed to generate a channel response estimatebased on a first received symbol signal; and a timing control moduledisposed to generate an FFT timing adjustment parameter based at leastin part on the channel response estimate, wherein the timing controlmodule comprises: a variable rate interpolator; and a fine symbol timingmodule disposed to generate, based at least in part on the channelimpulse response estimate, the FFT timing adjustment signal as an errorsignal, wherein said error signal is generated so as to adjust thesample rate of the variable rate interpolator responsive to the channelimpulse response estimate.
 14. The apparatus of claim 13, furthercomprising an FFT module, said FFT module dispose to receive an outputof the variable rate interpolator and generate an output signalapproximating a transmitted symbol corresponding to the received symbol.15. The apparatus of claim 13, wherein the fine symbol timing modulecomprises: an absolute value module configured to generate an absolutevalue signal based on the channel impulse response estimate; a peaklocator module configured to detect one or more peak values in theabsolute value signal; and an error calculation module configured togenerate the FFT timing adjustment signal.
 16. An apparatus forcontrolling FFT timing in a receiver, comprising: a signalcharacterization module disposed to generate a channel response estimatebased on a first received symbol signal; and a timing control moduledisposed to generate an FFT timing adjustment parameter based at leastin part on the channel response estimate, wherein the signalcharacterization module further comprises a clock error module disposedto generate a clock error signal.
 17. The apparatus of claim 16, whereinthe timing control module comprises a combiner module and a variablerate interpolator module, said combiner module configured to combine theFFT timing adjustment signal and the clock error signal so as togenerate, as an output, a combined error signal, said output coupled toan input of the variable rate interpolator.
 18. A method for adjusting avariable rate interpolator in an OFDM receiver, comprising: generating achannel impulse response estimate for a received OFDM signal;determining a location of one or more peak values in the channel impulseresponse estimate; generating a timing error signal based at least inpart on the one or more peak values; and adjusting the variable rateinterpolator responsive to the timing error signal.
 19. The method ofclaim 18, wherein the estimating the location of one or more peak valuesin the channel impulse response comprises: selecting a threshold value;determining one or more pairs of threshold crossing locations of thechannel impulse response estimate; and generating the location estimatesof the one or more peak values based on ones of corresponding one ormore pairs of threshold crossing locations.
 20. The method of claim 19,wherein ones of the location estimates of the one or more peak valuesare generated as the average of ones of the corresponding thresholdcrossing locations.
 21. The method of claim 18, wherein the generating atiming error adjustment signal comprises: comparing the one or morepeaks to a set of one or more reference values; selecting ones of theestimates of the one or more peaks that are within a predefined searchdistance WIN of corresponding ones of the one or more reference values;and generating the timing error adjustment signal as a function of thedifference between the selected ones of the one or more peak values andthe corresponding ones of the one or more reference position values. 22.The method of claim 21, wherein the function of the difference betweenthe selected ones of the one or more peak values and the correspondingone of the one or more reference position is the sum of the differences.23. The method of claim 18, wherein the generating the channel responseestimate for the OFDM signal is based on a pilot tone included in theOFDM signal.
 24. A communication apparatus comprising a processor moduleconfigured to: generate a channel impulse response estimate for areceived OFDM signal; determine a location of one or more peak values inthe channel impulse response estimate; generate a timing error signalbased at least in part on the one or more peak values; and adjust thevariable rate interpolator responsive to the timing error signal.
 25. Acommunication apparatus, comprising: means to generate a channel impulseresponse estimate for a received OFDM signal; means to determine alocation of one or more peak values in the channel impulse responseestimate; means to generate a timing error signal based at least in parton the one or more peak values; and means to adjust the variable rateinterpolator responsive to the timing error signal.
 26. A computerprogram product comprising a computer-readable medium including codesfor causing a processor to: generate a channel impulse response estimatefor a received OFDM signal; determine a location of one or more peakvalues in the channel impulse response estimate; generate a timing errorsignal based at least in part on the one or more peak values; and adjustthe variable rate interpolator responsive to the timing error signal.27. An apparatus for adjusting a variable rate interpolator in an OFDMreceiver, comprising: a channel impulse response estimator circuitdisposed to generate a channel impulse response estimate for a receivedOFDM signal; and a fine symbol timing (FST) circuit disposed to generatean error signal to be used at least in part to adjust the variable rateinterpolator, wherein the error signal is generated based on one or morepeaks of the channel impulse response estimate.
 28. The apparatus ofclaim 27, wherein the channel impulse response estimator comprises acircuit to generate the channel impulse response estimate based on apilot tone provided in an OFDM signal received by the OFDM receiver. 29.The apparatus of claim 28, wherein the FST circuit includes: a peaklocator circuit disposed to estimate the position of one or more peaksin the channel impulse response estimate; and an error determinationcircuit disposed to determine a position error between the estimate ofthe one or more peaks and one or more reference values.
 30. Theapparatus of claim 29, further comprising a updating circuit disposed toupdate and store the peak reference values and provide the updated peakreference values to the error determination circuit.
 31. The apparatusof claim 29, wherein the peak locator circuit is configured to estimatethe location of the one or more peaks in the channel impulse responseestimate by: selecting a threshold value; determining one or more pairsof threshold crossing locations of the channel impulse responseestimate; and generating the peak location estimates of the one or morepeak values based on corresponding one or more pairs of thresholdcrossing locations.
 32. The apparatus of claim 31, wherein ones of thepeak location estimates of the one or more peak values are generated asthe average of ones of the corresponding threshold crossing locations.33. The apparatus of claim 28, wherein the error signal is generated by:comparing the one or more peaks to a set of one or more referenceposition values; selecting ones of the estimates of the one or morepeaks that are within a search distance of corresponding ones of the oneor more reference position values; and generating the error signal as afunction of the difference between the selected ones of the one or morepeak values and the one or more reference values.
 34. The apparatus ofclaim 33, wherein the function of the difference between the selectedones of the one or more peak values and the one or more reference valuesis the sum of the differences.